The present invention relates generally to an acoustic inspection system, and more particularly, to a method and apparatus for ultrasonically inspecting continuous sheets in an on-line production basis.
Ultrasonic transducers are often used to inspect various articles in a non-destructive manner. Typically, these devices include a pulser/receiver which produces an electrical pulse that excites a piezoelectric or magnetostrictive transducer, causing it to emit an ultrasonic pulse. In the "pulse-echo" technique, this ultrasonic pulse travels into the article under inspection until it is reflected from an interface. The reflected pulse is received and converted by the same transducer into an electrical signal which is then amplified and conditioned by the receiver for further analysis. In some applications, the "through-transmission" technique is used where the signal is transmitted by one transducer and received by a second transducer; but the generation and reception of pulses is similar to the "pulse-echo" technique. The transducer output may be analyzed to provide information concerning imperfections in the article, and the results can be displayed on an oscilloscope or other output medium.
Ultrasonic transducers have been used to inspect continuous coextruded plastic sheet in order to detect the presence of a barrier layer therein. In such an application, it is necessary to provide a coupling medium between the face of the transducer and the near surface of the plastic sheet being inspected so that the face of the transducer will not reflect the majority of the ultrasonic energy. One known technique for accomplishing this is to provide a large vat containing an interfacing medium (e.g., water), immersing the face of the transducer in the medium, and then passing the plastic sheet through the medium beneath the transducer.
This technique requires that the portion of the sheet being inspected be fully submerged within the interfacing medium in the vat. The vat requires a substantial amount of space within the manufacturing facility which can usually be put to more effective use. Such inspection systems also frequently require that the sheet be bent or otherwise deformed so that it can be properly submerged in a reasonably sized vat. Bending the sheet is undesirable as this can easily cause the sheet to be damaged.
Ultrasonic transducer apparatus, having means for maintaining a coupling medium between the transducer and the portion of the surface of an article under inspection are also known. In U.S. Pat. No. 3,485,088, for example, an ultrasonic transducer apparatus is described which includes structure for maintaining a column of interfacing fluid between the face of the transducer and the surface of an article being inspected.
A serious problem with many apparatus of this type is that air bubbles tend to become entrapped in the fluid between the face of the transducer and the surface of the article under inspection, particularly, adjacent the face of the transducer. Any air bubbles which are present in this region, even bubbles of microscopic size, can cause substantial error in the accuracy and performance of the measurements.
In using transducer apparatus of this type to inspect extensive articles, it may be desirable to move the transducer apparatus and the article relative to each other to permit different portions of the article to be inspected. When the article being inspected is a coextruded plastic sheet or another article of relatively fragile nature, there is a substantial risk of abrading or otherwise damaging the surface of the article by the transducer apparatus.